In the manufacture of thick film circuits, a conductive ink composition is wiped across a print screen by a squeegee to form a conductor pattern on substrate underlying the print screen. Normally, a layer of dielectric is interposed between successive conductor print layers, but it has been proposed to use multiple successive conductor printing steps with no intervening dielectric. In certain cases, multiple printing is used to produce fine pitch features; in other cases, it is used to over-print as a means of improving reliability or producing extra thick or multiple composition conductor systems.
In the manufacture of single print thick film circuits, it is customary to print an alignment or registration feature in the margin of the substrate that can be optically inspected to measure the alignment of the print relative to the substrate or a previous print. Such optical alignment testing is time-consuming and is generally performed off-line on a random basis to detect gross alignment errors. Moreover, such optical alignment testing is fairly limited, as it cannot be used to detect other printing flaws such as excessive line spread, and localized print distortion due to stretching of the print screen, for example. These flaws are particularly significant in multiple print circuits because misalignment, localized print distortion and excessive line spread may result in inadvertent shorting between conductors, especially in fine pitch circuits.